Christine Wilson, 25, told the girl, who was under the legal age of consent, that her name was Chris before they became involved in sexual contact on various occasions.
The High Court in Edinburgh heard that afterwards the girl found out Wilson's real identity and that she had been conned into losing her virginity.
Police who were investigating Wilson discovered that it wasn't the first time that she had tricked a girl.
In 2008, Wilson had posed as a man to form a sexual relationship with another teenage girl, the court heard.
The girl, who was also 15 when they first met in Aberdeen, dumped Wilson after seeing a copy of the photo page of her passport.
Wilson, of Aberdeen, pled guilty to two charges of obtaining sexual intimacy from two girls.
Prosecution lawyer Jane Farquharson told the court that Wilson had problems with her sexuality since a young age. She said: "The accused, Christine Wilson, has by her own admission experienced some confusion about her sexuality.
"Both complainers believed they were in a relationship with a boy."
Ms Farquharson told the court that Wilson was introduced to her first victim by a mutual friend shortly before her 16th birthday in the summer of 2008. They exchanged telephone numbers and began texting each other.
Ms Farquharson added that the girl had at first refused to believe rumours that Chris was in fact a woman, but in August 2008 was contacted by a girl who emailed her a copy of Wilson's passport.
Wilson, when confronted, denied she was female but later admitted her pretence. The girl then ended their relationship, the court heard.
Wilson was interviewed by police in October 2011, and admitted to duping the girl to form the relationship.
The court also heard that Wilson met the second female at the end of March 2010 and they began sexual contact later in the year. The victim had told Wilson that she was aged 16.
The victim eventually discovered that Wilson was not a boy in August 2010 when police contacted her. Judge Lord Bannatyne deferred sentence for reports.